Recently, the consciousness of global-environment preservation has risen and measures against the warming effect have become important. In particular, automobile exhaust gases exert considerable influences on the environment and it is expected to clean the gases. It is generally known that to clean automobile exhaust gases is influenced by the performance of the motor vehicles and the composition of the gasoline fuel. The petroleum refining industry is especially required to provide a high-quality gasoline capable of contributing to automobile exhaust gas cleaning.
A gasoline is produced by blending gasoline blend-stocks obtained in crude oil refining steps. In particular, the FCC gasoline which is obtained by the catalytic cracking reaction of a heavy hydrocarbon oil is incorporated into gasolines in large quantities and exert a considerable influence on gasoline quality improvements.
The catalytic cracking of a heavy hydrocarbon oil is a reaction in which a heavy oil, which is a low-quality oil obtained in a petroleum refining step, is catalytically cracked to thereby convert it into a light hydrocarbon oil. However, hydrogen, coke, liquid petroleum gas (LPG), lighter fractions (light cycle oil: LCO), and heavy fraction (heavy cycle oil: HCO) are yielded as by-products in producing FCC gasoline. For efficiently producing FCC gasoline, persons skilled in the art prefer a catalyst which has high cracking activity and with which a high-quality gasoline having a high octane number is obtained in a high gasoline yield.
A method for obtaining high-quality FCC gasoline has been proposed in which a high-silica zeolite which has the high acidity, e.g., ZSM-5, is added to a catalyst to yield FCC gasoline having an increased light-olefin content and an improved octane number (see, for example, patent document 1). This method, however, has had a disadvantage that the yield of the FCC gasoline decreases. Furthermore, a method of catalytic cracking in which a heavy oil is converted to a light olefin fraction and FCC gasoline having a high octane number has also been proposed (see, for example, patent document 2). This method has a problem that coke deposits on the catalyst in an increased amount although the amount of olefins serving to heighten the octane number increases.
Patent Document 1: JP-A-61-82845
Patent Document 2: JP-A-10-195454